The invention relates to an alumino-silicate glass, which contains alkaline earth metals for lamp bulbs, which have molybdenum components and bulb temperatures of greater than 650° C., and its use.
High demands are placed on glasses for high-temperature lamps, which are generally understood to mean lamps having bulb temperatures of above 550° C. The glasses in question are alumino(boro)-silicate glasses which contain alkaline earth metals. The glasses must be essentially free of alkali metal oxides, since alkali metal ions interfere with the regenerative halogen cycle in the lamp. This is because during operation of the lamp there is a chemical equilibrium between formation and decomposition of tungsten halides in the tungsten vapor from the filament and the halogen/inert gas mixture. The decomposition reaction takes place at higher temperatures than the formation reaction, so that the tungsten is deposited back on the filament. If this cycle is interfered with by contaminating components, such as for example alkali metal ions, the tungsten is deposited not on the filament, but on the inside of the glass bulb, as an undesirable shiny black disruptive coating.
The patent literature already contains numerous documents, which relate to glasses for incandescent lamps. However, these glasses are afflicted with a very wide range of drawbacks.
U.S. Pat. No. 3,978,362 describe incandescent lamps with lamp bulbs made from glass which has a high CaO content (14-21% by weight).
Other documents claim glasses, which have specific ratios between CaO and BaO:
DE-B-27 33 169 relates to MgO-free glasses for seals with molybdenum in which the CaO:BaO weight ratio is between 0.6 and 1. These glasses are B2O3-free. Drawbacks of these glasses are: that when the lamp is operating they have an increased tendency to lamp blackening (deposits on the inner side of the bulb) and to the formation of white deposits (likewise on the inner side of the bulb). The glasses described here are also susceptible to crystallization during the production process.
DE 29 30 249 C2 relates to the use of specific glass compositions as bulb material, in which compositions the BaO:CaO weight ratio is between 2.3 and 3.5 (CaO:BaO between 0.28 and 0.43). The glasses described in the latter document are said to have an improved resistance to what is known as “reboil”. Reboil is the tendency of the glass to form a large number of small bubbles of included gases when subjected to further processing with a flame or when reheated. This impairs the light transmission and weakens the reheated areas.
U.S. Pat. No. 4,060,423 describes B2O3-free glasses for Mo seals in which the Al2O3/(BaO+CaO) weight ratio lies in the range from 0.6 to 1.
U.S. Pat. No. 4,298,388 also describe B2O3-free glasses for glass-Mo seals. These glasses are also MgO-free and have high CaO contents (up to 19.2% by weight). BaO is only an optional component.
DE 37 36 887 C2 describes B2O3-free, low-CaO glasses. These glasses have disadvantageously high working points.
Glasses, which require B2O3 for incandescent lamp bulbs are also known: For example, the glasses for seals with molybdenum described in U.S. Pat. No. 3,310,413 contain 4 to 9% by weight of B2O3. The sealing or bulb glasses described in DE 33 05 587 Al also require 3 to 7% by weight of B2O3 and, moreover, high BaO contents (11-16% by weight). Such high B2O3 contents, in particular in combination with MgO, reduce the viscosity values, so that these glasses are unsuitable for halogen lamps with bulb temperatures of greater than 650° C., for example of approx. 700° C. The low stability of the glasses at high temperatures leads to the lamp bulb bulging, possibly even to such an extent that the bulb will explode. One example of such a glass is the commercially available glass VI, having the composition (in % by weight) 56.8 SiO2; 16.4 Al2O3; 4.7 B2O3; 5.8 MgO; 7.8 CaO; 8.0 BaO, with an annealing point AP of 721° C.
The glasses disclosed in DE 197 58 481 C1 and DE 197 47 355 C1 also contain B2O3. They contain relatively small quantities of BaO.
U.S. Pat. No. 3,496,401 describes incandescent lamps made from an alumino-silicate glass containing alkaline earth metals and having a maximum alkali metal oxide content of 0.1% by weight, in particular made from glasses comprising SiO2, Al2O3, 10-25% by weight of alkaline-earth metal oxides, the specific levels of which are not given in further detail, and 0-10% by weight of B2O3. The exemplary embodiments are either B2O3-free or contain at least 4% by weight of B2O3. The maximum permissible alkali metal oxide content is too high for the high bulb temperatures, of approx. 700° C. and, during operation of the lamp, will lead to blackening of the internal surface of the bulb.
In order to be used as glass for lamp bulbs, which contain molybdenum components as electrode material or electrical lead material, the thermal expansion of the glass must be adapted to that of molybdenum, so that a tight, stress-free seal between the metal and the glass is achieved.
This means that the glass must have a higher expansion coefficient at its setting temperature {set point) than Mo, i.e. the difference in expansion between Mo and the glass must be positive, in order to achieve a radial compressive stress in the glass which is beneficial to the sealing of Mo supply leads.
A further demand placed on a glass, which is to be suitable for use as glass for lamp bulbs, is that it be suitable for tube drawing. For this to be so, it must have sufficient crystallization stability.